The high-capacity batteries used in electric or hybrid vehicles serve to store electric energy. In an electric vehicle, the battery is supplied with energy by connecting it to a power source. In hybrid vehicles, energy can additionally be recovered during braking of the vehicle.
In operation, that is on charging or decharging, or when the stored energy is taken from the battery, heat is released. Batteries possess an optimal operating temperature so that the developed heat must be eliminated, since an increased operating temperature causes a very heavy thermal load of the battery cells and electronic components. In addition, very high demands must be satisfied in relation to a low temperature spread between the individual battery cells.
Cooling the battery, that is to be seen as heat source, extends its life and should be carried out such that the temperatures of the cooled battery vary only within a limited range, and the temperature spread between individual battery cells is as small as possible.
A cooling device for the battery cannot be integrated into the engine cooling system of a hybrid vehicle due to too high temperatures of up to about 100° C. within the engine cooling system. The highest temperatures within the battery, dependent on the battery type, should be 40° C. to 60° C. maximum. Therefore it is necessary to use an additional cooling device.
It is known to cool batteries in electric or hybrid vehicles by use of the ambient air. But on a hot summer day, the temperature of the outside air can reach, or even exceed, 40° C., so that cooling based on unprepared ambient or outside air is not possible. On the one hand, it is conceivable to reduce the battery output at these outside conditions in order to limit the developed heat. But the disadvantage arises that the battery cannot provide its maximum output.
On the other hand, it is possible to take the cooling air either from the ambience or the air-conditioned passenger compartment and pass the air through the battery after having the air cooled by the air-conditioning unit of the vehicle.
Use of cooling air taken from the inside of the passenger compartment enables a narrower temperature range compared with the use of ambient air, but taking air from the passenger compartment increases the noise in the vehicle, hence reducing the passengers' comfort.
And when the batteries are air-cooled, a very high temperature spread between the individual battery cells as well may develop. To reduce the temperature spread, a very large air mass flow rate must be used for cooling. Further, when an air-based cooling system is used, in addition to the aforementioned loud flow-borne noise and the cooling output being dependent on the ambient conditions, large space requirements for air ducting and the necessary fans are extremely disadvantageous.
In addition to cooling the battery using air cooled by the air conditioning unit of the vehicle, other methods are known to link the battery cooling system to the air conditioning system of the vehicle. First, it is possible to directly cool the battery using the refrigerant, and second, by use of a secondary circuit of the air conditioning unit. In direct cooling, the heat exchanger is supplied with refrigerant to absorb the heat that develops within the battery. When cooling is based on a secondary circuit, the heat that is absorbed in the heat exchanger of the battery could be dissipated in a second heat exchanger to the air conditioning unit of the vehicle. Water or glycol, for example, could function as circulating heat carrier.
In DE 10 2006 004 419 A1, a cooling design is proposed that is provided with a cooling water circuit for cooling the heat-generating components. The cooling water circuit is connected to the engine cooling water circuit, for example, of a hybrid vehicle. The closed water circuit and a refrigerant circuit of the air conditioning unit of the vehicle are thermally coupled over a water/refrigerant heat exchanger. The heat absorbed in the water circuit as secondary circuit is transferred to the refrigerant circuit in the water/refrigerant heat exchanger.
From DE 10 2006 004 419 A1, a heat exchanger for absorbing the heat developed within the battery is known, where the plate-like heat-generating elements in a plate thickness direction are arranged such that predetermined distances between said elements are established as fluid channels. The heat-generating elements are firmly integrated into the heat exchanger that due to the wide flow cross-sections on the heat carrier side with external dimensions has a disadvantageously very large installation space. Owing to the transfer of sensible heat, additionally, dependent on the heat output to be eliminated a large mass flow of the heat carrier is necessary.
Other different devices and systems are proposed in prior art as heat exchangers for absorbing the heat developed within the battery.
In DE 198 49 491 C1, an electrochemical energy storage is disclosed that comprises a heat exchanger structure for temperature control of the storage cells. The heat exchanger structure is provided with two advance channels positioned on opposite sides of the storage cell arrangement and two return channels each corresponding on the opposite side. The accompanying parallel arranged channels are passed in opposite directions. The heat exchanger channels that are established as pipe elements with rectangular cross-sections also function to support the storage cells. The shape ensures good area contact between the channels and the rectangular storage cells. As tempering medium particularly, air, water or a similar liquid refrigerant is used.
From DE 10 2007 044 461 A1, a heat exchanger unit, that is established as waveguide cooler, for an electrochemical energy storage is known, which is provided with flow channels passable by a tempering medium. The channels are provided at their ends with feeding or collecting, respectively, advance distributing channels or return collecting channels that are arranged opposite to each other and with their flow cross-sections established rectangular. For an improved adaptation to circular storage cells, the flow channels are designed wave-shaped in direction of flow. A gaseous medium such as air or a liquid medium such as water is used as tempering medium.
The heat exchangers of the prior art are for cooling storage cells of a battery, especially a vehicle battery, owing to their operation mode based on air or water have wide flow cross-sections on the heat carrier side. Therefore, a disadvantageously large installation space of the heat exchanger, and hence, of the storage device is required. Owing to the transfer of sensible heat, a very large mass flow of the heat carrier is necessary as well, dependent on the heat output to be eliminated. The temperature increases in flow direction of the heat carrier within the heat exchanger so that no homogeneous temperature distribution within the storage cells can be ensured. Wide temperature spreads between the individual battery or storage cells, on the other hand, have a negative effect on the cell life.